1. Field of the Invention
The present invention relates to a development method in the field of electrophotography or electrostatic recording, particularly of such a type that electrostatic images are developed by the use of a one-component developer. More particularly, the present invention relates to a method for effecting development by coating a one-component toner on the endless surface of a resilient conductive supporting member in the form of a roller or belt which is rotatably or movably mounted, and then transferring the toner to an electrostatic image bearing surface.
2. Description of the Prior Art
Various types of developing method using a one-component developer are heretofore known such as the powder cloud method which uses toner particles in cloud condition, the contact developing method in which a uniform toner layer formed on a toner supporting member comprising a web or a sheet is brought into contact with an electrostatic image bearing surface to effect development, and the magnedry method which uses a conductive magnetic toner formed into a magnetic brush which is brought into contact with the electrostatic image bearing surface to effect development.
Among the above-mentioned developing methods using the one-component developer, the powder cloud method, the contact developing method and the magnedry method are such that the toner contacts both the image area to which the toner should be adhered and the non-image area or background area to which the toner should not be adhered and therefore the toner more or less adheres to the non-image area as well, thus unavoidably creating the so-called fog or background deposition.
To avoid such fog, there has been proposed the transfer development with space between a toner donor member and an image bearing member in which a toner layer and an electrostatic image bearing surface are disposed in opposed relationship with a clearance therebetween in a developing process so that the toner is caused to fly to the image area by the electrostatic field thereof and the toner does not contact the non-image area. Such development is disclosed, for example, in U.S. Pat. Nos. 2,803,177; 2,758,525; 2,838,997; 2,839,400; 2,862,816; 2,996,400; 3,232,190 and 3,703,157. This development is a highly effective method in preventing the fog or background deposition. Nevertheless, the visible image obtained by this method generally suffers from the following disadvantages because it utilizes the flight of the toner across the air gap resulting from the electric field of the electrostatic image during the development.
A first disadvantage is the problem that the sharpness of the image is reduced at the edges of the image. The state of the electric field of the electrostatic image at the edge thereof is such that if an electrically conductive member is used as the developer supporting member, the electric lines of force which emanate from the image area reach the toner supporting member so that the toner particles fly along these electric lines of force and adhere to the surface of the photosensitive medium, thus effecting development in the vicinity of the center of the image area. At the edges of the image area, however, the electric lines of force do not reach the toner supporting member due to the charge induced at the non-image area and therefore the adherence of the flying toner particles is very unreliable and some of such toner particles barely adhere while some of the toner particles do not adhere. Thus, the resultant image is an unclear one lacking sharpness at the edges of the image area, and line images, when developed, give an impression of having become thinner than the original lines.
To avoid this in the above-mentioned toner transfer development, the clearance between the electrostatic image bearing surface and the developer supporting member surface must be sufficiently small (e.g. smaller than 100.mu.) and actually, accidents such as pressure contact of the developer and mixed foreign substances are liable to occur between the two surfaces. Also, maintaining such a fine clearance often involves difficulties in designing of the apparatus.
A second problem is that images obtained by the above-mentioned toner transfer development usually lack half-tone reproducibility. In the toner transfer development, the toner does not fly until the toner overcomes the binding power of the toner supporting member by the electric field of the electrostatic image. This power which binds the toner to the toner supporting member is the resultant force of the Van der Waals force between the toner and the toner supporting member, the force of adherence among the toner particles, and the reflection force between the toner and the toner supporting member resulting from the toner being charged. Therefore, flight of the toner takes place only when the potential of the electrostatic image has become greater than a predetermined value (hereinafter referred to as the transition threshold value of the toner) and the electric field resulting therefrom has exceeded the aforementioned binding force of the toner, whereby adherence of the toner to the electrostatic image bearing surface takes place. But the binding power of the toner to the supporting member differs in value from particle to particle or by the particle diameter of the toner even if the toner has been manufactured or prepared in accordance with a predetermined prescription. However, it is considered to be distributed narrowly around a substantially constant value and correspondingly the threshold value of the electrostatic image surface potential at which the flight of toner takes place also seems to be distributed narrowly around a certain constant value. Such presence of the threshold value during the flight of the toner from the supporting member causes adherence of the toner to that part of the image area which has a surface potential exceeding such threshold value, but causes little or no toner to adhere to that part of the image area which has a surface potential lower than the threshold value, with a result that there are only provided images which lack the tone gradation having steep .gamma. (the gradient of the characteristic curve of the image density with respect to the electrostatic image potential).
In view of such problems, a developing device in which a pulse bias of very high frequency is introduced across an air gap to ensure movement of charged toner particles flying through the air gap, whereby the charged toner particles are made more readily available to the charged image is disclosed in U.S. Pat. Nos. 3,866,574; 3,890,929 and 3,893,418.
Such high frequency pulse bias developing device may be said to be a developing system suitable for the line copying in that a pulse bias of several KHz or higher is applied in the gap between the toner donor member and the image retaining member to improve the vibratory characteristic of the toner and prevent the toner from reaching the non-image area in any pulse bias phase but cause the toner to transit only to the image area, thereby preventing fogging of the non-image area. However, the aforementioned U.S. Pat. No. 3,893,418 states that a very high frequency (18 KHz-22 KHz) is used for the applied pulse voltage in order to make the device suitable for the reproduction of tone gradation of the image.
Moreover, in U.S. patent applications Ser. Nos. 58,434, the continuation of which matured into U.S. Pat. No. 4,395,476, on July 26, 1983 and 58,435, now U.S. Pat. No. 4,292,387, issued Sept. 29, 1981 assigned to the present assignee, it has been proposed that, in order to obtain better half-tone gradations of the image, an AC voltage of low frequency may be applied to a small air gap between the toner supporting member and the latent image bearing member to cause the toner particles to reciprocate in said air gap so that the latent images will be developed. These prior arts provide better one-component development methods.